Carburization of metal articles



F. R. ANDERSON Fil-ed nec. '22, 1955' CARBURIZATION OF METAL ARTICLESAug. 27, 1957 CARBURIZATION F METAL ARTICLES Floyd R. Anderson, Denver,Colo., assigner to Gardner- Denver Company, Denver, Colo., a corporationof Delaware Application December 22, 1955, Serial No. 554,767

6 Claims. (Cl. 148-19) This invention relates to a process for casehardening hollow metal members, and more particularly, to an improvedprocess for case hardening both the interior and exterior of suchmembers which may have diicultly accessible interior surfaces.

Although the instant invention may have application in a number of elds,particularly those involving the case hardening of both the interior andexterior surfaces of elongated hollow articles having relatively thinbody portions separating the interior and exterior surfaces, the instantinvention is particularly adapted to the formation of a novel drill rodstructure. The so-called rock drill rods have generally acceptedstandard sizes ranging from about 2 to about 20 feet in length and fromabout 0.75 to 2 inches in diameter. Actually, hexagonally crosssectionedrods are usually employed with maximum crosssection dimensions of 7minch up to 17/8 inches (but such' dimensions are considered to bediameters of a substantially round cross-section for the purposes of theinstant description). The rods are formed from steel stock by hot orcold working usually over a suitable mandrel, which results in theformation of an approximately 1A inch axial bore in the rods. Althoughthe particular manner in which the bore is formed in the rods is not soimportant, this bore does extend the entire length of the rod and theratio of bore length to bore diameter is very substantial. v

Heretofore, drill rods have been made of high carbon steels which aredifficult to work; but it has only recently been discovered that drillrods giving distinctly superior performances may be made from steelshaving a lower carbon content which are case hardened on both theoutside surface and the inside surface defining the axial bore. The caseon the interior surface is very important here, and it should be aboutequal in thickness to the exterior case. A number of difiiculties,however, have been encountered in attempting to fabricate such a casehardened drill rod.

cementing or carburizing step and then a'quenching step wherein the hotcarburized article is immersed in a quenching liquid such as water oroil so as to quickly cool the :article from substantially thecarburizing temperature to substantially room temperature (or at leastto a temperature of about 300 F. so that further metallurgical `changesdo not occur on cooling to room temperature). The carbon content of thecarburized surface portions as well `as the rate :at which such surfaceportions are quenched will determine the hardness and other desirableproperties in the case hardened layer. Differences in case thicknessesparticularly in the difiicultly accessible interior surfaces may causedefects. Also, different quenching or cooling rates for differentsurface portions may make such a significant difference in the resultingmaterial that flaws or defects may be created, if such surface portionsare adjacent. In addition, excessive warpage, over and above that whichmay be obtained using ideal carburizing and htates Patent i APatentedAug. 27, 1957 ice quenching techniques, may result from lack of uniformquenching.

A peculiarity of drill rods and similar hollow articles having diicultlyaccessible interior surfaces is uniform carburization of the interiorsurfaces is difficult and that uniform quenching of such surfaces bycontact with the quenching liquid cannot be obtained.

For example, the carburization operation per se presents a number ofproblems. Uniform carburization with gas has been found to bepractically impossible because of the difficulty in having a uniformconcentration of gas throughout the bore. Packing the rodsV in solidcarburizing material also presents difficulties not only in the tendencyfor slightly slower carburization (which is actually a gas-solidreaction) within the bore but also in the tendency to bend the rods sopacked because of shifting of the carburizing material duringcarburization. Liquid carburizing baths (KCN) present safety hazards.

As another problem, the initial quenching liquid entering the bore isonly a very small amount of liquid which engages :a rather substantialamount of heated surface and this results immediately in vaporization ofthe quenching liquid which, in the case of such drill rods, has beenfound to violently force vapor and liquid out of the Opposite end of thebore. Such a fast flowing stream of heated steam or oil which is veryhazardous and difficult tohandle is undesirable. In addition, thesurfaces of the bore are not uniformly contacted with liquid, because ofthe presence of vapor in the bore, so that certain of the difficultieshereinbefore mentioned are encountered.

The instant invention affords a unique solution to the problems peculiarto this art. First of all, the invention contemplates 'filling the boreof the rod with a normally solid carburizing material and sealing suchmaterial in the bore so that contact with the bore thereby atcarburizing temperatures will result in a uniform carburization at apredetermined rate. The rod itself is heated to carburizing temperaturesin 4a carburizing fluid which will, of course, be exposed only to theexterior surfaces of the rod :and which also carries out lcarburizationof such exterior surfaces uniformly and at a predetermined rate.Preferably, the rod is suspended from one end in the carburizing fluidto minimize bending or warpage. In addition, the concentration of thecarburizing material in the fluid can be controlled so that thecarburizing rate is controlled and the resulting case thicknesses on theinside and outside surfaces may have the desired ratio. Forexample,diffusion techniques may be employed to correlate the two carburizingrates in a situation wherein the solid carburizing material in the boreis somewhat slower. In such situation, carburization in a gas atmospherecan be carried out to the extent desired on the exterior surface of therod and the rod can then be maintained at carburization temperatures ina substantially inert atmosphere (or an atmosphere which will not causedecarburization) for an additional period of time, during whichdiffusion takes place along the exterior surface and additionalcarburization may take place in the bore of the rod.

Although quenching by -actual surface contact between the quenchingliquid and the surface to be case hardened has heretofore beenconsidered a necessary aspect of the procedure, the instant inventionadditionally provides a process which avoids this procedural step withrespect to the difficultly accessible interior surfaces of the article,while still obtaining case hardening thereof. In other words, theinstant invention contemplates quenching the rod by contacting theoutside surface only thereof with the quenching liquid, while preventingcontact between the quenching liquid and the inside surface. This isaccomplished by sealing the open ends of the bore before carburizationand retaining the seal during quenching;

and thev instant invention is based in part on the discovery that thesomewhat less' rapid cooling ofthe'interio'r sur? faces by heatconduction through the thinshell or body of the rod results in not onlyan extremely uniform but also'an effective case hardening'oftheinteio'rsu'r'face. The uniformity of the interior surface-casehardeninghas'A been found 'to be partcula'rl'y'important fromA aA'strength and performance point" of 'viewlr y `It is',` therefore, animportant" object" of the instant invention' to provide an improved casehardeng operation for hollow drill rods and the like'artic'les.'

It is a further object of the'in'stantinvention to provide animprovedmethod of producing a steel 'drill rod having" an axial borethat"comprises 'filling the 4bo're with particulatecarburizingrnaterial, heating the rod to carburizing temperaturev in thepresence of a carburizing fluid, lsealing the ends of thebore, andquenching the rod by contacting the'outs'ide only of the rod withquenching liquid, whereby cooling of the inside surface is accomplishedby heat conduction throughthe body of the rod.

Other and further objects, features and advantages of the presentinvention willV become apparent to those skilled in the art from`thefollowing detailed disclosure thereof and the drawings attached heretoand made a part hereof.

On the drawings:

Figure 1 is a sectional elevational View of a case hardened drill rodmade by the process embodying' the instant invention, and shown withsealing means closing the open ends of the bore;

Figure 2 is an enlarged cross-sectional view of the drill rod of Figure1 taken substantially along'the line II-II of Figure 1; and v Figure 3is a cross-sectional view taken substantially along the line III-III ofFigure 2.

As shown on the drawings:

In Figure l, the reference numeral 10 indicates generally a drill rodembodying the instant invention at the completion of the quenching step.The drill rod 10 has a case hardened layer 11 defining its outerperipheral surface, case hardened layers 12 and 13 defining,respectively, the top and bottom end surfaces of the rod 10 and a casehardened surface portion or layer 14 defining an axial bore B extendingthe full length ofthe rod 10. Actually, the rod 10 comprises a firstcase hardened sleeve 14 defining the axial bore B, a second vcasehardened sleeve 11 dening the' outer surface for the rod 10 and asteel-body 15 interposed between the rst and sec-` ond case hardenedsleeves 11 and 14. This structure ha's been found to be uniquelysatisfactory for drill rods.

As shown in Figure l, the overall length "l of standard drill rods mayrange from 2. to 20 feet, whereasv the maximum' cross-sectionaldimension or diameter D may be from 3A to2 inches and the diameter d'ofthe bore B is customarily about 1A inch. Although'it isy standardcommercial practice to employ a bore diameter d of approximately 1Ainch, it will be appreciated that this dimension d may range from aslittle as about s inch to as much as about 1/2 ich for the purposes ofthe instant invention. Because of the manner in which the bore B isusually formed during working of the steel, it is ordinarily notperfectly circular but only approximately so and the dimension drepresentsV generally the average diameter (or corresponding averagecross-sectional dimension).

As will be noted from Figure 2, the overall cross-section of the rod 10is hexagonal and the maximum cross-sectional dimension D is thedimension hereinbefore indicated and, for the purposes of the instantinvention, the dimension D may be considered to be a diameterof'asubstantially round cross-section (or substantially` the' averagecross-sectional dimension).

As will be seen, the ratio of the bore length-l to the' bore diameter(on the basis of the standard'bore diameter of 1A inch) ranges from aminimum of about'lOOl to' a maximum of ab'otl000:l (although the'maximumis best chilling or quenching effect on the case hardened layer 14 bythe instant quenching process. Wall thicknesses substantially greaterthan this dimension'will serve to slow down the cooling through the bodyof the rod to too substantial an extent to obtain the best results.

The first step in the practice of the instant invention is thecarburizing step wherein both the inside surface layer 14 of the rod andthe outside surface layer 11 of the rod are carburized. Carburizationper se (sometimes referred to as cementation) is a well known process,

which comprises exposing the surface to be carburized to a carburizingmaterial at carburizing temperatures. The carburizing material may besolid or fluid. For example, charcoal and/or coke in combination withchemical energizes in particulate compositions such as are described inRodman U. S. Patents Nos. 949,448 and Y 1,432,416 may be employed. Also,a bath of a salt containing carbon, such as a potassium cyanide saltbath may be employed as a liquid carburizing material; or a hydrocarbonsuch as natural gas may be employed asa gas carburizing material. whichmay be used in the practice of the instant invention the carbon beginsto penetrate the steel surfaces-at about 1300 F., but temperatures up toas much as 2000o F. may be used to accelerate the process. Preferably, a

y carburizing temperature of 1650 to 1700 M. is employed.

In the practice of the instant invention, the rst process step .is thatofffilling the bore B with particulate carburizing material M. Suchparticulate carburizing material may be in the form of potassium cyanidesalt particles (with or without other carburizing particulate materialssuchas thosementioned by Rodman) but if such particles of a saltcontaining carbon are to be used as an essential ingredient in theparticulate carburizing material which fills the bore B, it is necessaryto pack suchparticles in the bore rather tightly in order to assurecontact between the carburizing material and the bore walls 14 at thecarburizing temperatures. In other words, the saltsof this type melt atthe carburizing temperatures and, if packed loosely in the bore B, mightonly ll the lower portion of the bore B when melted. Preferably, theparticular carburizing material used is a refractory material or amaterial which remains solid at carburizing temperatures. By the use ofsuch refractory materials it is possible to maintain close contactbetween the carburizing material and the bore walls 14 in a uniformmanner throughout the bore B. Preferably, the particulate carburizingmaterial is a coked carbonaceous material such as coked charcoal, coal,etc., which contains a small amount of energizer (i. e. l to 20% byweight) such as an alkaline earth carbonate or an alkali metalcarbonate, which has been ground down to a ne particle size (e. g.suitable to pass an 8 mesh screen) so that it can be conveniently fedinto the bore B.

Referring to Figure l, it is generally preferable to' maintain the rod10 in an upright position, and a bottom steel plug 17 is force-lit intothe bottom of the bore B, and the particulate material is then pouredinto the bore B so'as to completely fill the same.y Then, the top steelplug 16 is force-fit'into the top of the bore B. At this point in theprocess, it will be appreciated that the carburized layers 11, 12, 13and 14 are not yet on the rod 10.

In the next step, the rod 10 is suspended by its upper end in thefurnace so as to hang freely.` This is of particular advantage since it'tends to eliminate a substantial In such carburizing processes amountof warpage in the rod 10 during carburation; and this is one of thereasons for employing a liuid carburizing material to carburize theouter layer 11 of the rod 10. In the furnace the rod 10 may be suspendedby its upper end in a fluid carburizing material such as a bath of amolten salt containing carbon (such as a potassium cyanide bath). Onedisadvantage of the use of potassium cyanide is the safety factor inview of the poisonous character of the fumes; but better control ofcarburization can also be obtained using a gas carburizing material asthe fluid carburizing material herein. An important feature of theinstant invention resides in the fact that the inner case thickness ordepth c and the outer case thickness or depth C may be controlledadequately during the instant operation, so the use of gas for a moreeasily controlled carburization of the outer layer 11 is of particularadvantage. Gases which may be used as carburizing fluids contain, as thecarbon bearing component, such materials as natural gas, volatilizedhigher hydrocarbons, etc.

The Vtime at which the ferrous metal surface is exposed to thecarburizing material at the carburizing temperature will, of course,determine the depth `of the carburized layer and the depth of theresultant case hardened layer. For the purposes of the instant inventionthe carburized or case depths c and C on the inside and outsidesurfaces, respectively (Figure 2) may range from about 0.010 to about0.125 inch for the practice of the instant invention, although thepreferred case depth c and/ or C for use in the practice of the instantinvention in the fabrication of drill rods is 0.050 to 0.090 inch(preferably c and C are 0.05-0.07 D). The carbon content in thesecarburized portions is Oil-1.2%.

As a typical example, using a 11/2 inch hexagonal crosssectioned SAE4320 steel, 20 foot drill rod having a 1A inch axial bore, the bore iscompletely filled with a solid carburizing material, in particulate formsuitable to pass an 8 mesh screen, composed of coked charcoal and/orcoal (100 parts by weight), barium carbonate (5 parts by weight) andsodium carbonate (5 parts by weight). The rod is then suspended freelyfrom one end thereof in a furnace, heated (in an inert atmosphere) to1700 F. then natural gas is fed into the furnace (at 200 cu. ft./hr.) tocreate a carburizing atmosphere; and the rod is maintained at 1700o F.for S hours which is followed by a 2 hour diffusion period at l700 F. inan inert atmosphere. This results in carburized layers 111 and i4 Vonthe outside and inside surfaces having carburized depths C and c each of0080-0090.

Comparable results are obtained by maintaining the same rod immersed ina molten potassium cyanide bath for the same period of time and at thesame temperature.

If either of the foregoing procedures is repeated except that a 7/8 inchhexagonal cross-sectioned SAE 8620 steel, 10 foot drill having a 1A inchaxial bore is employed at a carburizing time of 41/2 hours and adiffusion time of 11/2 hours, the resulting carburized outside andinside depth C and c are each 0.05-0.06 inch.

The sealing of the opposite ends of the axial bore B, in each case, maybe accomplished merely by forceiitting plugs 16 and 17, respectively(Figure 1) therein. Such plugs may be made of steel or other suitablyrefractory material, but are preferably made lof steel so as to stand upunder the shock of the quenching operation. Quenching in the customarymanner from the carburizing temperature (or from a temperature of atleast about l500 F.) is carried out by plunging the carburized rods intoquenching oil (or optionally Water). During the quenching, the rods arequickly cooled to a temperature which may be considered to roomtemperature, in that the temperature is low enough (perhaps 300 F. orless) that further metallurgical changes do not take place on cooling toroom temperature.

After quenching of the carburized rods just described, the plugs areremoved; and it is found that the tensile strength of the body is about125,000 p. s. i. whereas 6 that of the case hardened layerslll and 14 isabout 300,000 to 400,000 p. s. i; (andthe Rockwell C hardness is 55-63,as compared to 3.5-43 in the starting material).

The resulting rods are then straightened by (comparatively easy) coldworking to eliminate the slight warpage, if any; and the rods may begiven customary subsequent heat treatments to remove strains, etc.

Although the instant invention may be employed using any ferrousmaterials, certain materials are preferred for drill rods. As mentioned,heretofore, drill rods have been made of high carbon steels such as SAE1080 (specification: O15-0.88% C, 0.60 0.90% Mn, 0.040% P maximum,0.050% S maximum, remainder Fe). The steels preferred for use in thepractice of the invention have relatively low carbon contents of0.l0-0.30% C and may contain 0.40-l.00% Mn (plus 0.040% P max., 0.040% Smax., and O20-0.35% Si); but these steels also contain small amounts ofone or more elements such as Ni, Cr and Mo which retard the martensite(or which serve to retain at least 20-30% austenite in thecarburized rodinside surface upon quenching). The amounts of such elements areG40-3.75% Ni, 0.40- l.l0% Cr and/or 0.15-0.30% Mo. The preferred steelfor use in the invention is SAE 4320 (or AISI 4320), specification:0.17-0.22% C, OAS-0.65% Mn, 0.040% P max., 0.040% S max., O20-0.35% Si,l.65-2.00% Ni, 0.40-0.60% Cr max., O20-0.30% Mo, in the case of mostsizes of drill rod stock; but for the smaller size stock of one inchdiameter or less SAE 8620 (or AISI 8620) is preferred and this has alower Ni content and slightly higher Mn content, specification:0.18-0.23% C, U70-0.90% Mn, 0.040% P max., 0.040% S max., 0.200.35% Si,0.40-0.70% Ni, G40-0.60% Cr and 0.l5-0.25% Mo. rThe additional Mn in SAE8620 compensates for the reduced Ni in retarding martensite formation.

lt will be understood that modifications and variations may be effectedwithout departing from the spirit and scope of the novel concepts of thepresent invention.

I claim as my invention:

l. A method of carburizing a steel drill rod having an axial bore thatcomprises filling the bore with particulate carburizing material,closing off the ends of the bore to retain the particulate materialtherein, sealing the ends of the bore, suspending the rod from one ofits ends, heating the rod to carburizing temperature in the presence ofa carburizing fluid, and then quenching the rod by contacting theoutside surface only with quenching liquid, while retaining the seal ofthe ends of the bore.

2. A method of carburizing a steel drill rod having an axial bore thatcomprises filling the bore with particulate carburizing material,sealing the ends of the bore, heating the rod to carburizing temperaturein the presence of a carburizing fluid, quenching the rod whileretaining the seal of the ends of the bore, and then removing the sealand the particulate carburizing material from the bore.

3. A method of carburizing a ferrous metal article having an elongatedhole therein, which comprises filling the hole with particulatecarburizing material, sealing the material therein,heating the articleto carburizing temperature in the presence of a carburizing uid, wherebycarburization of the surface defining the hole and the outside surfaceof the article is accomplished, and quenching the article by contactingthe outside surface only with quenching liquid, while retaining the sealon the particulate material in the hole.

4. A method of carburizing a ferrous metal article having an elongatedhole therein, which comprises filling the hole with particulatecarburizing material that remains solid at carburizing temperature,sealing the material therein, heating the article to carburizingtemperature in the presence of a carburizing fluid, wherebycarburization of the surface defining the hole and the outside surfaceof the article is accomplished, and then quenching the article bycontacting the outside surface only with quenching liquid,-whileretaining' the seal on the-particulate material inthe hole.A

5. A method of carburizing a steel drill rod having an axial bore thatcomprises filling the bore with particulate carburizing material,closing off the ends vof the bore to 5 `of` a carburizinguid,and.'quenching the rod by .conretain the particulate materialtherein, sealing the endsV tactingtheoutsideonly ofthe rod withyquenching liquid, of the bore, suspending the -rod` from onev of.' itsends, wherebycooling' of theinside surface is accomplished by heatingthe rod to carburizing temperature inthe presheatcon'ductionlthrough'the body of the rod, said steelv ence of a carburizing fluid,and quenching the rod by rod havin'g'the followinglcompositiont'contacting the outside only of the rod with quenching 10 018;() 23% C' o40 0 70% Ni liquid, whereby cooling ofthe inside surface is accom- 0'700`90% Mn 0'40 0`60% Cr plished by heat conduction through the body ofthe rod, 0'0407'0 -P *maxi 0'15 0'25% `M0 said steel rod-having thefollowing composition: 0'040% AS Remail'lder. Fe` 0.l7-0.22% C1.65-2.00% Ni 15 0.20-0.35%Si 0.45-0.65% Mn OAG-0.60% Cr 0.040% P max.O20-0.30% Mo References Cited in the'ile'of this patent 0.040% S max.Remainder Fe UNITED S'Ifyntsy PATENTS O20-0.35% Si 1,459,409 McQualdJune 19, 1923 6. A method of carburizing a steel drill rod having-an `202,398,809 f Snyder Apr. 23, 1946 axial bore that comprises lling thebore with particulate carburizingmaterial,-'closing ot the ends ofsthebore to retain thetparticulate material therein, sealing the ends of,the' bore,-suspending the rod'from one of its ends,y

heating the rod to-carburizing temperature in the presence

1. A METHOD OF CARBURIZING A STEEL DRILL ROD HAVING AN AXIAL BORE THATCOMPRISES FILLING THE BORE WITH PARTICULATE CARBURIZING MATERIAL,CLOSING OFF THE ENDS OF THE BORE TO RETAIN THE PARTICULATE MATERIALTHEREIN, SEALING THE ENDS OF THE BORE, SUSPENDING THE ROD FROM ONE OFITS ENDS, HEATING THE ROD TO CARBURIZING TEMPERATURE IN THE PRESENCE OFA CARBURIZING FLUID, AND THEN QUENCHING THE ROD BY CONTACTING THEOUTSIDE SURFACE ONLY WITH QUENCHING LIQUID, WHILE RETAINING THE SEAL OFTHE ENDS OF THE BORE.